(a) Field of the Invention
The invention relates to a cDNA probe for human methylenetetrahydrofolate reductase (MTHFR), and its uses.
(b) Description of Prior Art
Folic acid derivatives are coenzymes for several critical single-carbon transfer reactions, including reactions in the biosynthesis of purines, thymidylate and methionine. Methylenetetrahydrofolate reductase (MTHFR; EC 1.5.1.20) catalyses the NADPH-linked reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, a co-substrate for methylation of homocysteine to methionine. The porcine liver enzyme, a flavoprotein, has been purified to homogeneity; it is a homodimer of 77-kDa subunits. Partial proteolysis of the porcine peptide has revealed two spatially distinct domains: an N-terminal domain of 40 kDa and a C-terminal domain of 37 kDa. The latter domain contains the binding site for the allosteric regulator S-adenosylmethionine.
Hereditary deficiency of MTHFR, an autosomal recessive disorder, is the most common inborn error of folic acid metabolism. A block in the production of methyltetrahydrofolate leads to elevated homocysteine with low to normal levels of methionine. Patients with severe deficiencies of MTHFR (0-20% activity in fibroblasts) can have variable phenotypes. Developmental delay, mental retardation, motor and gait abnormalities, peripheral neuropathy, seizures and psychiatric disturbances have been reported in this group, although at least one patient with severe MTHFR deficiency was asymptomatic. Pathologic changes in the severe form include the vascular changes that have been found in other conditions with elevated homocysteine, as well as reduced neurotransmitter and methionine levels in the CNS. A milder deficiency of MTHFR (35-50% activity) has been described in patients with coronary artery disease (see below). Genetic heterogeneity is likely, considering the diverse clinical features, the variable levels of enzyme activity, and the differential heat inactivation profiles of the reductase in patients' cells.
Coronary artery disease (CAD) accounts for 25% of deaths of Canadians. Cardiovascular risk factors (male sex, family history, smoking, hypertension, dyslipoproteinemia and diabetes) account for approximately 60 to 70% of the ability to discriminate CAD patients from healthy subjects. Elevated plasma homocysteine has also been shown to be an independent risk factor for cardiovascular disease.
Homocysteine is a sulfhydryl-containing amino acid that is formed by the demethylation of methionine. It is normally metabolized to cysteine (transsulfuration) or re-methylated to methionine. Inborn errors of metabolism (as in severe MTHFR deficiency) causing extreme elevations of homocysteine in plasma, with homocystinuria, are associated with premature vascular disease and widespread arterial and venous thrombotic phenomena. Milder elevations of plasma homocysteine (as in mild MTHFR deficiency) have been associated with the development of peripheral vascular disease, cerebrovascular disease and premature CAD.
Homocysteine remethylation to methionine requires the folic acid intermediate, 5-methyltetrahydrofolate, which is produced from 5,10-methylenetetrahydrofolate folate through the action of 5,10-methylenetetrahydrofolate reductase (MTHFR). Deficiency of MTHFR results in an inability to metabolize homocysteine to methionine; elevated plasma homocysteine and decreased methionine are the metabolic consequences of the block. Severe deficiencies of MTHFR (less than 20% of activity of controls) as described above, are associated with early-onset neurologic symptoms (mental retardation, peripheral neuropathy, seizures, etc.) and with atherosclerotic changes and thromboembolism. Milder deficiencies of MTHFR (35-50% of activity of controls), with a thermolabile form of the enzyme, are seen in patients with cardiovascular disease without obvious neurologic abnormalities.
In a survey of 212 patients with proven coronary artery disease, the thermolabile form of MTHFR was found in 17% of the CAD group and 5% of controls. In a subsequent report on 339 subjects who underwent coronary angiography, a correlation was found between thermolabile MTHFR and the degree of coronary artery stenosis. Again, traditional risk factors (age, sex, smoking, hypertension, etc.) were not significantly associated with thermolabile MTHFR. All the studies on MTHFR were performed by enzymatic assays of MTHFR in lymphocytes, with measurements of activity before and after heat treatment to determine thermolability of the enzyme.
Since 5-methyltetrahydrofolate, the product of the MTHFR reaction, is the primary form of circulatory folate, a deficiency in MTHFR might lead to other types of disorders. For example, periconceptual folate administration to women reduces the occurrence and recurrence of neural tube defects in their offspring. Neural tube defects are a group of developmental malformations (meningomyelocele, anencephaly, and encephalocele) that arise due to failure of closure of the neural tube. Elevated levels of plasma homocysteine have been reported in mothers of children with neural tube defects. The elevated plasma homocysteine could be due to a deficiency of MTHFR, as described above for cardiovascular disease.
Neuroblastomas are tumors derived from neural crest cells. Many of these tumors have been reported to have deletions of human chromosome region 1p36, the region of the genome to which MTHFR has been mapped. It is possible that MTHFR deletions/mutations are responsible for or contribute to the formation of this type of tumor. MTHFR abnormalities may also contribution to the formation of other types of tumors, such as colorectal tumors, since high dietary folate has been shown to be inversely associated with risk of colorectal carcinomas.
MTHFR activity is required for homocysteine methylation to methionine. Methionine is necessary for the formation of S-adenosylmethionine, the primary methyl donor for methylation of DNA, proteins, lipids, neurotransmitters, etc. Abnormalities in MTHFR might lead to lower levels of methionine and S-adenosylme-thionine, as well as to elevated homocysteine. Disruption of methylation processes could result in a wide variety of conditions, such as neoplasias, developmental anomalies, neurologic disorders, etc.
Although the MTHFR gene in Escherichia coli (metF) has been isolated and sequenced, molecular studies of the enzyme in higher organisms have been limited without the availability of an eukaryotic cDNA.
It would be highly desirable to be provided with a cDNA probe for human methylenetetrahydrofolate reductase (MTHFR). This probe would be used for identification of sequence abnormalities in individuals with severe or mild MTHFR deficiency, including cardiovascular patients and patients with neurologic symptoms or tumors. The probe would also be used in gene therapy, isolation of the gene, and expression studies to produce the MTHFR protein. The probe would also provide the amino acid sequence of the human MTHFR protein, which would be useful for therapy of MTHFR deficiency by biochemical or pharmacological approaches.
It would be highly desirable to be provided with a molecular description of mutations in methylenetetrahydrofolate reductase deficiency.
Patients with sequence abnormalities in MTHFR might have different responses to drugs, possibly but not limited to drugs that affect folate metabolism. Therefore, it would be useful to know if these mutations are present before determining the appropriate therapy. The drugs/diseases for which this might be relevant include cancer chemotherapeutic agents, antibiotics, antiepileptic medication, antiarthritic medication, etc.